If you already browsing the internet for some time I bet you already found error 404 not found error.
The 40x error refers to the error caused by the client like typing the nonexistence URL, below is the list of 40x errors:
The most common 40x error are:
400 Bad Request
The server does not understand the command issued by the user, for example, you type the wrong URL in the internet browser.
Request to a URL need authorization, when you accessing a web page that requires a password
402 Payment Required
Reserved for a future, not used yet.
URL is correct but the server does not allow us to access it
404 Not Found
The webpage does not exist, this is usually when a document deleted from the web server
405 Method Not Allowed
The requested method does not allow in the server
406 Not Acceptable
407 Proxy Authentication Required
Need authentication from the proxy server before accessing a document
408 Request Timeout
Request out of time window provides by the server, the client can try again
The request cannot be processed usually because of simultaneous changes on the same file.